Twinning-induced Plasticity Steel Research Articles

Nonmetallic inclusion characterizations and effects on impact toughness anisotropy of TWIP steel with different Ce additions

The effects of cerium (Ce) addition (0, 0.015% and 0.045%) on the microstructure, inclusions and impact toughness of Fe-18%Mn-0.6%C twinning-induced plasticity (TWIP) steel were investigated. The results show that the addition of Ce has no significant effect on the texture of TWIP steel. The grain size of TWIP steel decreases, and the volume fraction of twins increases slightly with Ce addition. Typical MnO-MgO-MnS complex inclusions and MnS were observed in TWIP steel without Ce. The inclusions gradually transform from the long strip MnS-Ce oxysulfide complex inclusions to ellipsoidal Ce oxysulfides with the increase in Ce addition from 0.015% to 0.045%. Notably, the transverse impact toughness of TWIP steel with 0.015% Ce addition is much lower than that of the other samples and exhibits serious anisotropy, and these differences are attributed to the stronger deformation ability of the complex inclusions with MnS and Ce oxysulfides compared with Ce oxysulfides. Fine grain size and ellipsoidal inclusions lead to excellent isotropic properties in TWIP steel with 0.045% Ce addition.

The influence of the heat generation during deformation on the mechanical properties and microstructure of the selected TWIP steels

The TWIP (Twinning Induced Plasticity) steels are one of the most promising materials in reducing the weight of vehicles. Despite a lot of research on TWIP steel, there are some issues that are not explained enough. Due to the future use of TWIP steel and the manufacturing of the final part by metal forming, three issues still need to be clarified. The first one, which is the most important, is the increase of the temperature due to the conversion of the deformation work into heat. TWIP steel has a high limit strain, strength and lower conductivity than conventional steel, therefore the heat generation of TWIP steel is greater than for other materials. The second and third issues are combined. They concern the influence of V microadditions on the stress–strain curves, the strain hardening coefficient n and the strain rate sensitivity coefficient m under cold deformation conditions. These properties determine the cold formability of TWIP steels. In the research, two TWIP steels were used with and without V microadditions (MnAl and MnAl-V steel). The special methodology using strain and temperature measurement systems as well as light and scanning electron microscopy (SEM) were applied. Research shows a significant increase of the temperature in the material due to high plastic deformations as well as a high level of yield stress. In the neck area, for the highest strain rate of 0,1 s -1, at the moment of rupture, the temperature reaches more than 200 °C. The difference between the average temperature in the rupture area and the maximum temperature is equal to 100° C. Its high increase can lead e.g. to changes in the deformation mechanism from twinning to dislocation gliding, which is also connected with a worsened workability, and thus also energy consumption of the bodywork elements. MnAl-V steel has better or similar ductility for the deep drawing in comparison to MnAl steel at low strain rates for almost isothermal conditions (constant temperature during deformation). However the MnAl steel has better ductility for the larger strain rates over 0.1 s−1 then there is large heat concentration in a very narrow area for MnAl-V steel. The obtained results are very important from an application point of view. The strain rate sensitivity coefficient m of the steel MnAl has very low, and even negative, values, which can make the production of complicated drawpieces difficult. Higher values of the strain rate sensitivity coefficient are exhibited by steel MnAl-V, i.e. at the level of 0,05, which is almost constant in the whole range of the obtained deformations.

Material Properties of High-strength High Chromium TWIP Steel with Increased Corrosion Resistance

Twinning induced plasticity (TWIP) steels are a class of high-strength steels that have been devel-oped for their outstanding ductility and strength properties. TWIP refers to the fact that these steels display an unusually high degree of deformation before fracture due to the formation of twins during deformation. TWIP steels could be used in a variety of industries for structural applications or com-ponents that need to withstand high levels of stress and deformation. This article deals with the de-velopment of high strength with fully austenitic microstructure and high chromium content. Micro-structure, mechanichal and corrosion properties of this steel were studied.

Hydrogen-induced hardening of a high-manganese twinning induced plasticity steel

High-manganese twinning-induced plasticity (TWIP) steels exhibit high strain hardening, high tensile strength, and high ductility, which make them attractive for structural applications. At low tensile strain rates, TWIP steels are prone to hydrogen embrittlement (HE). Here though, we study the hardening and strengthening resulting from electrochemical hydrogen-charging of a surface layer of a Fe-26.9Mn-0.28C (wt.%) TWIP steel. We observed a 20% increase in yield strength following the electrochemical hydrogen-charging, accompanied by a reduction in ductility from 75% to 10% at a tensile strain rate of 10−3s−1. The microstructural evolution during tensile deformation was examined at strain levels of 3%, 5% and 7% by electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI) to study the dislocation structure of the hardened region. As expected, the microstructure of the hydrogen-hardened and the uncharged regions of the material evolve differently. The uncharged areas show entangled dislocation structures, indicating slip from a limited number of potentially coplanar slip systems. In contrast, hydrogen segregated to the grain boundaries, revealed by the deuterium-labelled atom probe tomography, delays the dislocation nucleation by blocking dislocation sources at the grain boundaries. The charged areas hence first show the formation of cells, indicating dislocation entanglement from more non-coplanar slip systems. With increasing strain, these cells dissolve, and stacking faults and strain-induced ε-martensite are formed, promoted by the presence of hydrogen. The influence of hydrogen on dislocation structures and the overall deformation mechanism is discussed in details.

Microstructure and mechanical properties of a novel twinning induced plasticity steel with two different grain morphologies

A novel heterogeneous-structure (HS) twinning-induced plasticity (TWIP) steel containing alternating columnar grain (CG) and equiaxed grain (EG) domains was successfully fabricated via prestraining, partial recrystallization, and directional solidification. Compared with the traditional EG sample, the HS sample exhibited a better tensile strength–elongation combination (yield strength ≈ 263 MPa, ultimate tensile strength ≈ 573 MPa, total elongation ≈ 101.5%). The mutually constrained structure could effectively control the unique deformation modes of the EG and CG domains at different deformation stages. The HS sample exhibited stable and continuous plastic deformation owing to the timely release of localized high strain or stress, postponed plastic instability, and restrained crack propagation owing to the constrained structure. The stress relaxation was due to twinning and the effects of geometrically necessary dislocations, which accommodated deformation incompatibility. The improved mechanical properties of the HS sample were due to the persistent occurrence of the TWIP effect under very low to high strain levels during the entire deformation process, the effective grain boundary hardening in the EG domain, and the additional work hardening provided by the GNDs.

The mechanism of twin thickening and the elastic strain state of TWIP steel nanotwins

A Twinning Induced Plasticity (TWIP) steel with a nominal composition of Fe-16.4Mn-0.9C-0.5Si-0.05Nb-0.05V was deformed to an engineering strain of 6%. The strain around the deformation twins were mapped using the 4D-STEM technique. Strain mapping showed a large average elastic strain of approximately 6% in the directions parallel and perpendicular to the twinning direction. However, the large average strain comprised of several hot spots of even larger strains of up to 12%. These hot spots could be attributed to a high density of sessile Frank dislocations on the twin boundary and correspond to shear stresses of 1–1.5 GPa. The strain and therefore stress fields are significantly larger than other materials known to twin and are speculated to be responsible for the early thickness saturation of TWIP steel nanotwins. The ability to keep twins extremely thin helps improve grain fragmentation, i.e. the dynamic Hall–Petch effect, and underpins the large elongations and strain hardening rates in TWIP steels.

Effect of pre-strain on high cycle fatigue property of TWIP steel

A commonly used material in vehicle structure is twinning-induced plasticity (TWIP) steel. In addition, during auto safety structural parts process, the material is pre-strained to different levels. Therefore, the influence of pre-straining on fatigue life needs to be examined for design and life assessment. Based on the current test data, due to the different types of steel and loading methods, the fatigue life of the material will show an increase or decrease with the increase of pre-strain level. The results of this study are compared with the recommended S-N fatigue, and the applicability and conservative levels of fatigue life curves are discussed to consider the influence of material pre-strain on the fatigue life assessment of automotive structural components.

Transmission Kikuchi Diffraction Mapping Induces Structural Damage in Atom Probe Specimens.

Measuring local chemistry of specific crystallographic features by atom probe tomography (APT) is facilitated by using transmission Kikuchi diffraction (TKD) to help position them sufficiently close to the apex of the needle-shaped specimen. However, possible structural damage associated to the energetic electrons used to perform TKD is rarely considered and is hence not well-understood. Here, in two case studies, we evidence damage in APT specimens from TKD mapping. First, we analyze a solid solution, metastable β-Ti-12Mo alloy, in which the Mo is expected to be homogenously distributed. Following TKD, APT reveals a planar segregation of Mo among other elements. Second, specimens were prepared near Σ3 twin boundaries in a high manganese twinning-induced plasticity steel, and subsequently charged with deuterium gas. Beyond a similar planar segregation, voids containing a high concentration of deuterium, i.e., bubbles, are detected in the specimen on which TKD was performed. Both examples showcase damage from TKD mapping leading to artefacts in the distribution of solutes. We propose that the structural damage is created by surface species, including H and C, subjected to recoil from incoming energetic electrons during mapping, thereby getting implanted and causing cascades of structural damage in the sample.

Effect of ECAP processing temperature on an austenitic TWIP steel's microstructure, texture and mechanical properties

The development of microstructure and texture of twinning-induced plasticity (TWIP) steel sheet during ECAP at two temperatures (250° and 350°) up to four passes were investigated. The high chromium content is the differential of this TWIP steel, a chemical element that reduces the stacking-fault energy and increases corrosion resistance. By imposing ECAP, the volume fraction of twins, dislocation density, and yield strength significantly increase while ductility decreases. This study compared the effect of temperature during applied severe plastic deformation (SPD). Thin parallel slip bands’ high density was observed, and in addition to an intense micro shear, banding took place when pressing was continued by increasing the number of passes. The martensitic transformation was not detected in the deformation-twinning microstructure after more passes. The relationship between the microstructure, strain-hardening behavior, and mechanical properties was investigated after two different deformation schedules of ECAP. The best balance between high strength and good ductility (1915 MPa and 7%) was obtained after four passes at 250 °C and 350 °C. It was found that the quantity of ECAP pass affects the displacement substructure with the formation of shear bands, subgrains, and various variants of twins. These also influenced strain-hardening behavior, microhardness, texture, and ultimate strength.

Unusual grain-size effects on tensile deformation behavior of twinning-induced plasticity steel with low Mn content

In this work, the effects of grain refinement on the tensile properties, dynamic strain aging (DSA) and twinning behavior of Fe–16Mn-0.6C steel, which has relatively low manganese content, were investigated using a room-temperature monotonic tensile test, OM, XRD, EBSD and TEM. Results show that the yield strength, tensile strength and elongation of the steel increase with the decrease of grain size. Grain refinement can significantly increase the serration amplitude and local strain concentration of DSA, thereby enhancing the DSA-strengthening effect. The microstructure observations show that with the formation of plentiful twins, the twinning capacity of coarse-grained steel is exhausted prematurely at initial strains. Although the twinning behavior of fine-grained steel is retarded in the early stage of deformation, a large number of fine and dense twinning structures can be produced continuously at high strains. This unique twinning behavior and enhanced DSA in the fine-grained steel maintain the strain-hardening capacity of the steel, which delays the plastic instability and hence brings about simultaneous increases in strength and ductility.

In-situ TEM investigation of deformation mechanisms of twinning-induced plasticity steel

In-situ TEM investigation of deformation mechanisms of twinning-induced plasticity steel

Mechanical characterization and microstructure of fiber laser welded TWIP980 steel depending on welding speed and focal distance

The TWIP steels are preferred in the automotive industry due to excellent strength and ductility. In this study, the effect of fiber laser welding parameters has been investigated on the mechanical properties and microstructural points of view twinning-induced plasticity (TWIP) steel. The variation of the mechanical properties depending on fiber laser welding parameters has been determined. The samples were laser-welded with different speeds and focal distances the laser power constant at 4 kW. The stress and elongation decreased when the welding speed increased. The highest stress was obtained at + 1 focal distance. The best energy absorption was seen in b-3 combination (laser power 4 kW, welding speed 130 mm/s, focal distance +1 mm).

Multi-Criteria Decision Making Methods for Selection of Lightweight Material for Railway Vehicles.

This paper deals with the selection of the optimal material for railway wagons, from among three different steel and three aluminium based materials, by using four different Multicriteria Decision Making Methods (MCDM) and comparing their ranking of the materials. We analysed: Dual-Phase 600 steel, Transformation-Induced Plasticity (TRIP) 700 steel, Twinning-Induced Plasticity (TWIP) steel, Aluminium (Al) alloys, Al 6005-T6, and Al 6082-T6, and porous Al structure with closed cells. Four different MCDM methods were used: VIKOR, TOPSIS, PROMETTHEE and the Weighted aggregated sum product assessment method (WASPAS). Key material properties that were used in the MCDM analysis were: density, yield strength (Y.S.), tensile strength (T.S.), Y.S./T.S. ratio, Youngs modulus (Y.M.), cost and corrosion resistance (C.R.). Research results indicate that aluminium and its alloys prove to be the most suitable material, based on setup criteria. Advanced steels also achieved good ranking, making them a valid option, immediately behind lightweight aluminium alloys. Porous aluminium did not perform well, according to the used MDCM methods, mainly due to the significantly lower strength exhibited by the porous structures in general.

The Influence of High Vanadium and Phosphorus Contents on the Risk of Transverse Cracking during the Continuous Casting of Austenitic TWIP Steels

There is considerable interest in improving the resistance of fully austenitic TWIP steels to hydrogen embrittlement; one potential route is to use V additions to promote hydrogen trapping by V(C,N) precipitates. This has the dual benefit of increasing the yield strength through precipitation strengthening and grain refinement. However, the effect on slab quality during continuous casting has not been determined. In this study, the hot ductility of two twinning-induced plasticity (TWIP) steels, Fe-0.6C-22Mn and Fe-0.6C-22Mn-0.2V, was examined over the temperature range 650–1200 °C. Tensile samples were taken from continuous cast 225 mm slabs and from 36 mm transfer bars. The addition of V caused the ductility trough in the temperature range 650–900 °C to deepen and widen and the lowest reduction in area (RA) recorded in the as-cast condition was 30%. This deterioration of hot ductility was due to V(C,N) precipitation. Even though the minimum RA was below the value often accepted to avoid cracking, no transverse cracking was observed in industrial trials and the surface quality was acceptable. The RA values of Fe-0.6C-22Mn were found to be very sensitive to the P level. However, this sensitivity was less evident when V was added, possibly due to P trapping by VC at austenite boundaries. No transverse cracking was observed in industrially produced slabs with P in the range examined (0.02 to 0.04 wt.% P).

Microstructural factors dictating the initial plastic deformation behavior of an ultrafine-grained Fe–22Mn-0.6C TWIP steel

While leading a great strain hardening capability, carbon-containing twinning-induced plasticity (TWIP) steels exhibit serrations on their stress-strain curves, resulting in barriers to commercial development. Although grain refinement is believed to suppress the serrations, how the grain size, particularly in the ultrafine-grained (UFG) range, and its orientation impacting on the serrations and plastic deformation mechanism are overlooked. Here, we compared the plastic deformation behavior in fine-grained (2 μm) and ultrafine-grained (0.86 μm) specimens, in both macroscopic and microscopic behavior, using digital image correlation (DIC) and scanning transmission and transmission electron microscopy (S/TEM) techniques. Our results showed that the dominant plastic deformation mode was changed from dislocation gliding and tangling to stacking faults and deformation twinning in the grains equal to or smaller than 1 μm (ultrafine grains). This alteration is also strongly influenced by the grain orientation, i.e., the maximum resolved shear stress for slip versus twinning. The enhancement of strain localization and the inhibition of the serrations in the UFG specimens are discussed.

Discussion of ceramic bar reinforced TWIP steel composite structure

Abstract A ceramic–steel composite structure which consists of Si3N4 ceramic bars and twinning-induced plasticity (TWIP) steel matrix was prepared using lost foam casting method. Microstructural analysis indicated that the ceramic bars and TWIP steel matrix can form favorable and tight interface. A comparison of mechanical properties between the composite structure and TWIP steel matrix was made. Results show that the bending strength increased from 388 to 805 MPa compared with TWIP steel matrix. The influence of annealing heat treatment on the bending strength of this new type of composite structure was also discussed.

Comparison of Temperature Distribution between TWIP and Plain Carbon Steels during Wire Drawing

The effect of the thermal properties of steels on wire drawing behavior has been investigated to understand and improve the wire drawing process. Finite element analysis and experimental tests were conducted to analyze the temperature distribution of the deformed specimens with different thermal properties. The thermal properties of twinning-induced plasticity (TWIP) steel were measured and compared with those of plain carbon steel. Based on the measurement of thermal properties, wire drawing behaviors were systematically compared with thermal conductivity of the specimen (k) using plain low-carbon steel with high k and TWIP steel with low k. The results revealed that the k of TWIP steel was approximately one third of that of low-carbon steel, and the thermal expansion coefficient of the TWIP steel was approximately 50% higher than that of low-carbon steel in the temperature range of 26-400 °C. The temperature distributions in the wire strongly depended on the k of the wire during wire drawing. TWIP steel exhibited higher maximum temperature, and took a longer time to attain the equilibrium temperature than low-carbon steel during wire drawing owing to the low k. The maximum temperature of the die increased with decreasing k of the wire, indicating that die wear can increase with decreasing k of the wire. Therefore, reducing the drawing speed is suggested for a wire with low k, such as high-alloyed metals, especially for TWIP steels.

Numerical simulation of a rapid fatigue test of high Mn-TWIP steel via a high cycle fatigue constitutive law

The generation of reliable data in the high cycle fatigue domain is crucial to support further metallurgic developments of fatigue optimized steel grades. Commonly employed for this aim, traditional standardized characterization methods are expensive and time-consuming. Thus, to circumvent these limitations, different accelerated fatigue testing methodologies have been proposed. In this work, the rapid fatigue test based on stiffness evolution is numerically reproduced using the finite element method for a specific grade of twinning-induced plasticity steel. A high cycle fatigue constitutive law grounded on the continuum damage mechanics framework is employed for this purpose. To adequately capture the material non-linear behavior observed in the experiments, a novel hardening–softening stress–strain curve for damage is proposed. The entire load history in the fatigue domain is modeled. A cycle-jump algorithm is used to improve the computational efficiency of the simulations. It is shown that a reduction of about 55% in the analysis elapsed time is reached by using this algorithm, while the result accuracy is maintained. Finally, the good agreement between numerical and experimental results, revealed by a maximum relative error smaller than 6.0%, evidences the potential of the present constitutive formulation to model the behavior of metals in the high cycle fatigue domain.

Effect of Elongation of a Rod on Strain Inhomogeneity and Shape Change During the Compression-Type Forming Process

Abstract The influence of elongation on the strain inhomogeneity and shape change in twinning-induced plasticity steel rod is systematically investigated to understand the macroscopic shear band (MSB) formation mechanism and to decrease the strain inhomogeneity during the compression-type forming processes. Specimens fabricated by rod flat rolling with elongation (3D rod) and by plane compression without elongation (2D rod) are compared using both finite element analysis and experiment. Despite the similar final product shape, the 2D rod presents a lower effective strain at the surface region than the 3D rod, leading to a high strain inhomogeneity. The higher effective strain at the surface region of the 3D rod is mainly attributed to the elongation of the 3D rod during the rolling. In contrast, the 2D rod exhibits strong dead metal zones owing to the lack of elongation of the specimen. Therefore, the formation of MSBs or strain inhomogeneity of a specimen can be reduced by increasing the elongation of the specimens during the forming process. Both the contact width and lateral spread of the 3D rod are lower than those of the 2D rod because of the elongation of the 3D rod originating from the slip effect at the rod–roll interface during the rolling process. The small frictional effect at the rod and roll interface increased the elongation of the rod, leading to a decrease in the strain inhomogeneity and lateral spreading in the 3D rod.

Effect of heat input on the microstructure and mechanical properties of the simulated coarse-grained heat-affected zone (CGHAZ) of novel twining-induced plasticity(TWIP) steel

The study of strengthening mechanisms associated with the HAZ is of significant interest for improving the mechanical properties. In this study, the effect of different heat inputs (6.21, 13.89, 19.64, 27.78, and 36.75 KJ·cm -1 ) on the microstructure, carbide precipitation, and mechanical properties of CGHAZ in a novel FeCMnSi TWIP steel was explored by using welding thermal simulations. The results showed that the CGHAZ organization did not experience phase transformation after different welding thermal cycles. As the heat input increased, and the grain size was 95.87, 107.03, 109.65, 98.68, and 136.95 μm, respectively. The factor of grain size variation with heat input is the dissolution of AlN particles and the generation of carbides at grain boundaries. The dramatic deterioration of CGHAZ toughness at a heat input of 27.78 KJ·cm -1 is mainly attributed to the enrichment of carbon and silicon elements at the grain boundaries in the CGHAZ, which leads to cracks along the grain boundaries. At this time, the fracture mode of CGHAZ shifts from ductility to intergranular brittleness. It is proposed that fine grain strengthening is the main factors in improving strength, and carbide precipitation is responsible for the significant decrease in toughness in the novel FeCMnSi TWIP steel. It provides an effective guide for the application of FeCMnSi TWIP steels.